Field of the Invention
The present invention relates to a substrate processing apparatus that performs processing of a substrate such as a semiconductor wafer, a glass substrate for a liquid crystal display device, or a glass substrate for a photomask (hereafter simply referred to as “substrate”) with a processing liquid and, in particular, is useful as a technique of performing concentration control while sensing the concentration of the processing liquid at a high precision when immersing and processing the substrate in the processing liquid that has been heated to a high temperature.
Description of the Related Art
Conventionally, as a substrate processing apparatus of this kind, there is known, for example, a substrate processing apparatus that performs an etching treatment of a silicon nitride film (SiN) on a substrate surface using a high-temperature solution of phosphoric acid (H3PO4).
For example, in the case of etching a silicon nitride film with a phosphoric acid solution in a semiconductor wafer process, it is general that a silicon oxide film (SiO2) is present on the substrate surface in addition to the silicon nitride film. In such a case, the object of etching is only the silicon nitride film, and it is generally demanded that the silicon oxide film is hardly etched by the processing liquid.
As a mechanism by which the silicon nitride film and the silicon oxide film are etched with a phosphoric acid solution, it is known that the silicon nitride film is etched by water in the phosphoric acid solution, and the silicon oxide film is etched by phosphoric acid in the phosphoric acid solution.
For this reason, in order to manage the etching amount of the silicon nitride film with a good precision, the concentration of the processing liquid, that is, the ratio of mixing a chemical agent liquid with a diluent, and the temperature of the processing liquid are extremely important. Also, generally, in a phosphoric acid solution at a high temperature, the amount of evaporation of water which is the diluent is large, so that concentration control by replenishment with water is important. As a substrate processing apparatus equipped with means for holding the temperature or concentration of the processing liquid to be constant, apparatus disclosed in JP-A-11-200072 and JP-A-2004-221540 are known.
For example, in JP-A-11-200072, there is disclosed a substrate processing apparatus that performs automatic control of the replenishment rate of water in accordance with a result of comparison between the current temperature and the boiling temperature of a phosphoric acid bath which is used as a set temperature in performing concentration control of the phosphoric acid bath by replenishment with water. In this apparatus, what is actually controlled is only the temperature of the processing liquid, and this is means for replenishment with the diluent that is dependent on the physical phenomenon that the concentration of the processing liquid does not become the boiling concentration or below.
However, in the case of the apparatus disclosed in JP-A-11-200072, since the phosphoric acid bath concentration corresponding to the boiling temperature of the phosphoric acid bath is constant, the concentration and the temperature of the processing liquid are simultaneously determined when the etching rate of the silicon nitride film is determined, thereby raising a problem in that the two cannot be independently adjusted. Also, there is a need to perform the replenishment so that the amount of replenishment with the diluent will be slightly larger than the original amount of evaporation; however, when the amount of replenishment becomes excessive, it will be a cause of decrease in the temperature of the processing liquid or bumping of the processing liquid.
Also, JP-A-2004-221540 discloses a substrate processing apparatus that includes temperature controlling means for operating the heating means so that the temperature of the processing liquid will be the set temperature, replenishment means for replenishing the processing tank with the diluent so as to adjust the concentration of the processing liquid, concentration sensing means for sensing the concentration of the processing liquid by the specific weight of the processing liquid or the like, and concentration controlling means for adjusting the amount of replenishment with the diluent so that the sensed concentration of the processing liquid will be a little higher than the boiling temperature.
However, in the case of the apparatus disclosed in JP-A-2004-221540, the concentration sensing means converts the pressure of the gas supplied from the sensing end within the processing tank to the specific weight of the processing liquid, so that the pressure of the gas supplied from the sensing end changes in accordance with the temperature of the processing liquid, the amount of liquid within the processing tank, and the flow of the processing liquid within the processing tank. Therefore, the sensing must be carried out under conditions such that the temperature of the processing liquid, the amount of liquid within the processing tank, and the flow within the processing tank are constant. However, in actual use, when the processing liquid is replenished with the diluent in order to control the concentration of the processing liquid, the temperature of the processing liquid also changes, thereby raising a problem in that a correct concentration cannot be determined until the temperature of the processing liquid returns to the set temperature.
Also, JP-A-2009-58306 discloses a concentration measuring method that calculates the concentration of metal ions in an aqueous solution of phosphoric acid by gradually lowering the temperature of the aqueous solution, optically sensing the change in transmittance by deposition of insoluble components, and determining the concentration of the metal ions from the deposition temperature, as well as an etching apparatus equipped with a reproduction system using the same.